The invention relates to machine tools, and more particularly to devices and methods for the contactless detection of a processing electrode of a machine tool.
Spark erosion machines are used to machine work pieces by means of electrical spark discharge between an electrically conducting workpiece and a processing electrode. In particular, the workpiece can be cut by means of a wire electrode or it can be machined by means of a bar-type cavity sinking electrode in that it is drilled or hollowed. In this process, in addition to removing material particles from the work piece the processing electrode itself also wears off. Consequently, new electrode material has to be supplied continuously to the work area of the spark erosion machine. In a known wire spark erosion machine, for example, the wire electrode moves from a dispenser roll via several pulleys and conveyor lines to the top wire guiding arm from where the wire electrode is moved via additional pulleys and a brake roll that controls the tensile stress of the wire electrode in the work area to the top wire guiding head where a power supply unit is provided for supplying the processing current.
The wire electrode continues from the top wire guiding head through the work area (where a work piece is mounted for machining) to the bottom wire guiding head from where it is guided into the disposal area via another pulley. While the work piece is being machined the wire electrode moves through the wire moving system at a speed of approximately 100-300 mm/sec.
Inserting the wire electrode or reinserting a broken wire electrode into the electrode moving system is a difficult and time-consuming job. The manufacturers of machine tools always try to design user-friendly products and to automate such operations so as to allow the operating personnel to focus on programming and supervising the system. In modern wire spark erosion machines the inserting process is at least partially automated, for example by means of using nozzles that spray a stream of fluid into a conveyor tube, thus pulling in the wire electrode and moving it along.
In order to be able to automate the insertion and movement of the wire electrode in the electrode moving system as fully as possible it is desirable to always know the current position of the electrode tip so as to activate or deactivate suitable moving means at the appropriate time and set suitable control parameters for the wire moving system. Also, insertion errors are easier to diagnose and correct with this method. Therefore, any required manual intervention by the operating personnel can be reduced to a minimum. xe2x80x9cElectrode tipxe2x80x9d should be understood to mean both the beginning and the end of the wire electrode as it can be very useful to know where the remainder of the wire electrode is located, for example when the wire breaks or when it is cut so as to quickly reinsert it and minimize the system downtime.
In micro-drilling, the position of the electrode end can be detected and, based on this detection, the remaining length of the electrode is determined. Hereinafter, the term electrode tip will be used for such cases as well.
In the prior art, various detection methods are used to detect the electrode tip. For example, U.S. Pat. No. 4,412,118 specifies a device for detecting the position of the wire tip when the wire breaks or when the wire is cut intentionally. In this case, the wire electrode is rewound after the interruption whereby it touches a sliding contact. A comparator circuit detects when the contact between the sliding contact and the wire electrode stops and discontinues rewinding the electrode. A variant is also disclosed where the wire is detected contactless by means of a photo sensor.
Similar wire detection methods are disclosed in U.S. Pat. No. 5,019,684 and U.S. Pat. No. 5,268,551 where a sliding contact or a photo sensor are also used for wire detection. The device specified in U.S. Pat. No. 5,019,684 additionally measures the length of the return path while the electrode is rewound and based on this it determines the location of the wire break.
Another wire detector, which is based on electrically contacting the wire electrode, is disclosed in U.S. Pat. No. 5,523,545.
For completeness"" sake German Patent DE 28 26 270 C2 should also be mentioned. This document specifies a different device not related to detecting the wire end. It relates to detecting and compensating the wire deflection caused by the spark erosive process forces (i.e. the deflection of the wire electrode on a plane that is vertical relative to the direction in which the wire advances). The proposed wire position sensor includes four measuring electrodes each of which is spaced apart from the wire electrode in one of the four axial directions +x, xe2x88x92x, +y and xe2x88x92y. A dielectric having a conductivity of 1-100 xcexcS/cm flows through the overall arrangement. The deflection of the wire electrode in the xy direction is measured in that the change in the resistance is detected between the four measuring electrodes and the wire electrode serving as a common electrode for all four measuring cells.
In the detectors known from the prior art which are based on sliding contacts it was found to be disadvantageous that the detector is in continuous contact with the processing electrode which could adversely affect the electrode moving system. Also, the sliding contacts are susceptible to corrosion and subject to wearing and tend to develop insulating oxide layers. For this reason sliding contacts are unsuitable for low sensor voltages in the wet area of an erosion machine. Even in a largely automated wire moving system of a wire spark erosion machine, for example where a wire electrode is moved via wire guiding tubes and rerouting units, a non-contact detection is advantageous. Although photo sensor arrangements are found in the prior art for contactless wire detection, the contactless detection has to be traded off against various other disadvantages, depending on the structural design of the photo sensors. Because the electrode wire moves continuously, dirt, paraffin, metal and metal oxide deposits have to be expected which contaminate the photo sensor and, thus, cause malfunctions.
It is also known that when the wire is moved via a fluid through a substantially closed tube system it is possible that air bubbles will develop. Optical sensors can be affected by such air bubbles, and they could cause it to operate improperly. Furthermore, a wire detector is frequently subjected to the transport fluid which is under pressure so that the photo sensors have to meet special requirements with regard to tightness. Therefore, optical sensors overall are relatively susceptible to malfunctioning. An ideal wire detection device should be able to detect the full spectrum of wire and bar-type electrodes available on the market. Therefore, it has to be highly sensitive and cover a considerable measuring range because it has to be able to work across the full cross-section of the moving system. Any optical sensors meeting the above requirements are very expensive.
A device is disclosed for detecting a processing electrode which is compact and cost-effective. The device is for the contactless detection of a bar-shaped or wire-shaped processing electrode of a machine tool (e.g., a spark erosion machine), having a measuring area through which the processing electrode can be moved. At least one measuring electrode is disposed in the measuring area. The detection is based on an impedance change in the measuring area which is caused by the processing electrode moving through the measuring area.
Furthermore, a wire spark erosion machine is proposed which is equipped with detection devices in multiple places along the moving path of the wire electrode so as to monitor the wire moving system, thereby allowing the wire moving system to be fully monitored.
Additionally, a method is provided for the contactless detection of a bar-shaped or wire-shaped processing electrode of a machine tool (e.g., a spark erosion machine), in a measuring area through which the processing electrode can be moved. An impedance change is detected in the measuring area by at least one measuring electrode disposed in the measuring area when the processing electrode moves through the measuring area.
xe2x80x9cDetectionxe2x80x9d not only means the differentiation between the xe2x80x9cprocessing electrode presentxe2x80x9d status and the xe2x80x9cprocessing electrode not presentxe2x80x9d status, it advantageously also includes measuring the accurate position and/or other characteristics of the processing electrode.
The disclosed devices and methods allow a wide spectrum of processing electrodes to be detected. Without requiring the problematic contact with the processing electrode such electrodes may have varying diameters, cross-sections and be made of different materials. The described detection method is preferably suitable for monitoring the wire moving system of wire spark erosion machines, but the method can just as advantageously be used in cavity sinking spark erosion machines (e.g., drilling spark erosion machines, milling spark erosion machines or micro cavity sinking spark erosion machines) because the processing electrodes used in these types of system are substantially bar-shaped.
Therefore, while the preferred primary application is the detection of electrically conducting wires or bars, the disclosed method is suitable for any measured object, including non-conducting objects, that cause a detectable change in the impedance (for example the resistive and/or capacitive component within the measuring area). Furthermore, the disclosed devices can be produced cost-effectively and do not require much space in the measuring area.
The measuring area is preferably filled with a transport fluid for the processing electrode. This fluid has an electrical conductivity that differs from the electrical conductivity of the processing electrode. Consequently, the impedance in the measuring area changes when the processing electrode moves through in that a portion of the fluid is displaced and replaced by the material of the processing electrode having a different conductivity. A transport fluid is usually used, especially in wire spark erosion machines, for inserting the processing electrode into the electrode moving system. At least while it is being inserted, which is a procedure during which the processing electrode can easily jam, stick or otherwise be misguided making it especially useful to monitor the electrode moving system, the moving path of the processing electrode is always filled with the transport fluid which can then also be used for the measurement.
The resistive component (hereinafter referred to as resistance) of the impedance change in the measuring area is analyzed. The measuring area is preferably filled with a transport fluid whose electrical conductivity is considerably lower than that of the processing electrode, but having a certain minimum conductivity of 1 xcexcS/cm, for example. Therefore, the resistance in the fluid-filled measuring area is relatively high as long as the processing electrode is absent. When a processing electrode with a high electrical conductivity moves through the measuring area, the total resistance in the measuring area decreases considerably as a path having a lower resistance is now present through a portion of the measuring area in the form of the processing electrode.
Two or more measuring electrodes are preferably disposed so as to be spaced apart in the direction in which the processing electrode advances to allow measuring the position of the processing electrode along the advancement path as accurately as possible. Alternatively, one measuring electrode is disposed in the measuring area while a second measuring electrode is formed by the processing electrode itself.
In a preferred embodiment, the measuring electrodes are ring-shaped. The device then preferably comprises two ring-shaped measuring electrodes spaced apart in the direction in which the processing electrode advances, and defining a cylindrical measuring area through which the processing electrode can be moved in an axial direction. The ring-shaped configuration of the electrodes is especially advantageous as the measured impedance is largely independent of the radial position of the processing electrode within the rings.
However, the measuring electrodes can also have other configurations. For example, they can be pin-shaped, U-shaped or assembled from several partial measuring electrodes. When the moving path is configured so as to be open (for example as a U-shaped channel), U-shaped measuring electrodes are advantageously used so as to keep the access to the channel unobstructed. As compared to optical detectors, the measuring detectors have the advantage that they are easily adaptable to the cross-section of the channel.
In another example, the capacitive component of the impedance change in the measuring area is evaluated. In this embodiment, two measuring electrodes, for example, cover a large portion of the measuring area, thus forming a type of capacitor whose capacity changes when the processing electrode moves through. In this case, it is advantageous that an insulating transport fluid can also be present between the processing electrode and the measuring electrode(s) (for example, air or a hydrocarbon). Furthermore, the measuring electrodes are not required to be in direct contact with the fluid in the measuring area. They are preferably mounted on the outside of an insulating protective sleeve encompassing the measuring area.
In still another example, the detection is substantially based on detecting the change in the inductive component of the impedance in the measuring area. A conducting processing electrode has a line inductance (albeit low), which can be detected based on an accurate measurement of the current in the measuring area, for example. In an alternative variant, the wire electrode in combination with the transport fluid flowing via a bypass forms a short-circuited secondary winding of a transformer where the induced secondary current changes depending on the proportion of the wire electrode in the electric circuit. The transmission of the measurement to the primary side is purely inductive. This variant is very robust electrically and mechanically, and it is also suitable for detection during erosion machining.
The detection of the processing electrode can preferably also be based on the evaluation of a combination of the resistive, the capacitive and/or the inductive components of the impedance change. For example, a combination of the inductive and resistive detection methods is advantageous.
During the measurement, an alternating voltage is preferably applied to one or more measuring electrode(s) and the capacitive, inductive and/or resistive component(s) of the impedance in the measuring area is determined based on the measured current. An alternating voltage source has the advantage that the measuring electrodes will not be damaged by the effects of electrolysis, electrophoresis or corrosion even in continuous operation.
The inside diameter of the device is preferably substantially equal to the inside diameter of the remaining electrode moving system in the machine tool as the device should not cause an obstruction when the processing electrode is inserted. Inserting the electrode is very difficult, especially in wire spark erosion machines, as the electrode consists of a wire having a width of only 10-350 xcexcm. Therefore, mistakes can easily occur when the electrode is inserted. The transport fluid, with which the wire electrode is typically moved through the wire moving system should also flow unobstructed through the detection device so as not to cause any turbulence or a pressure drop in the measuring area.
On the other hand, the measuring sensitivity of the device will increase if the detection device has a smaller inside diameter. In order to solve this conflict of interests, it is proposed to preferably configure the inside diameter of the detection device substantially smaller than the inside diameter of the remaining electrode moving system, while leading a portion of the transport fluid around the measuring area by means of a bypass.
In a preferred method, the impedance measured by a first detection device in a first measuring area is compared with the impedance in the measuring area of a second device through which the processing electrode does not move, but which is filled with the same fluid as the first device. As long as no processing electrode is present the two measuring devices will provide the same impedance value. When the processing electrode moves through the first measuring area the first device detects an impedance change while the impedance in the second measuring area will always remain constant. Therefore, no absolute impedance measurement is required for detection, only a comparative measurement. As a result, the impedance does not need to be measured with high absolute accuracy, and the necessity for a periodic calibration of the detection device is eliminated. The method can also be applied if the second detection device outside the moving path of the electrode is structurally not exactly identical with the first detection device. The varying characteristics of the two detection devices have to be recorded only once, and they have to be taken into consideration for detection by means of a conversion factor. The so-called comparator sensor method is characterized by an especially high resistance against interfering outside effects.
The disclosed devices not only permit the position of the processing electrode to be detected with high accuracy (a few millimeters, for example), it is also possible to measure other properties of the processing electrode. For example, the diameter of the processing electrode is preferably determined based on the impedance change when the processing electrode moves through the measuring area. When the geometric dimensions of the detection device are known the diameter of the processing electrode, which is the only unknown parameter, can be determined based on the conductivity of the transport fluid and the conductivity of the electrode material.
If the diameter of the processing electrode is also known, the surface quality of the processing electrode can also be determined based on the impedance change when the processing electrode moves through the measuring area. The surface quality (e.g., a contamination of the processing electrode, such as wax), can have a highly adverse effect on the spark erosion process. Such contamination can be detected because it insulates the processing electrode against the outside. The measurement of the resistive component of the impedance change would, therefore, result in only a minor drop in the resistance when the processing electrode moves through the measuring area.
Positioning the detection devices in various places of the wire moving system of a wire spark erosion machine permits the overall wire path to be monitored, and predetermined wire moving strategies can be triggered depending on the measured position of the wire electrode.